592 



INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



G 13-14. SEEBBA DE MAZAPIL, ZACATECAS. 



Burckhardt'"^* thus describes the Eo-Cretaceous and " Meso-Cretaceous " of 

 the northern part of Zacatecas : 



The Cretaceous limestones that overlie the shaly formations which we have just described 

 represent the complete series from the upper limit of the Jurassic to the Upper Cretaceous. 

 Near the naiddle of their thickness these limestones are divided into two parts by a very fossil- 

 iferous bed of marl known as the " Parahoplites bed." Furthermore, there is at the base of the 

 sequence a second marly bed, which is also rich in fossils, the " Holcostephanus bed." By 

 means of these fossiliferous marls, it is possible to establish four subdivisions of the Lower and 

 Middle Cretaceous of Mazapil. The lower marls, which contain Holcostephanus, are rather thin 

 and separated from the Jurassic by thin strata of gray limestone. 



These marls are generally yellowish and contain many nodules of iron oxide. They alter- 

 nate with beds of grayish or brownish limestone. Fossils are very numerous."^ * * * 



Above the marls with Holcostephanus there follow Hmestones which have a thickness of 

 400 to 500 meters and which are generally markedly distinguished by intercalations of chert. 

 These cherts, which are usually brownish at the surface and bluish in the interior, are commonly 

 of very irregular forms, and never assume a notably horizontal extent, as is the case with the 

 layers of chert in the Middle Cretaceous. It is very difficult to extract any fossils from the rock, 

 and the only fairly good specimen which I have found is a fragment of Holcodiscus. * * * 



Between the hmestones which have just been described and those of the Middle Cretaceous, 

 there occurs an argillaceous terrane, which may readily be recognized throughout the Sierra de 

 Santa Rosa. * * * The beds are not thick, but very characteristic — marls and marly lime- 

 stones* of a bright-yellow color, alternating with compact limestones which are grayish, bluish, 

 or slightly brownish. They everywhere contain many fossUs." * * * 



The Middle Cretaceous is represented by a very thick series of grayish limest-ones, which are 

 generally well stratified and alternate with layers of black chert. These limestones attain a 

 thickness of 400 to 500 jneters and are easy to recognize, as there is no other division of the 

 Jurassic or Cretaceous of Mazapil which offers so great a thickness of well-stratified Hmestones 

 in alternation with such regular beds and lenses of black chert. The intercalations of chert have 

 almost always great extent. They are strata rather than lenses. The rocks are not poor in 

 fossils. On the contrary, the beds abound in open ammonites, generally rather slender, belonging 

 to the genera Hamites, Crioceras, Ancyloceras, and probably Hamulina. But these remains 

 are not specifically determinable. 



Biirckhardt's table is given in Chapter XIII (pp. 552-553) . 



H 13. SOXJTHEASTEBN ABIZONA AND SONOBA. 



In the Bisbee district of Arizona Comanche strata overlap on Paleozoic^ accord- 

 ing to Ransome/^*'' who gives the following section: 



Generalized section of the Cretaceous rocks of the Bishee quadrangle, Arizona. 



- For list see the work cited. — B. W. 



